1. Field of the Invention
The present invention relates generally to a polishing slurry of chemical mechanical polishing (CMP), and more particularly to a closed loop concentration control system for polishing slurry of chemical mechanical polishing.
2. Description of the Prior Art
Integrated circuit (IC) complexity has continued to evolve, placing increasingly more demanding specifications on the processes used in their manufacture. As the requirement for increasing the density of active devices on an individual chip has escalated, the requirement for greater flatness, over long distances and short distances, on the surfaces, top and bottom, of the wafer has also evolved. Consequently, in the fabrication of integrated circuits, semiconductor substrate surface planarity is of extreme importance. In addition, flatness improves ability to fill via holes and lines through apertures in the dielectric.
Various processes have been used for planarization. One such process known as Chemical Mechanical Polishing (CMP) is presently being used in the most demanding applications. Chemical mechanical polishing (CMP) is the only technology, which can provide a total planariztion for the ultra-large scale integration (ULSI) process. This technology comes from IBM and has been developed through many decades, and been already applied on many products, such as to the central processing unit (CPU). The philosophy is the planarization technique which use a xe2x80x9cknife grinderxe2x80x9d like mechanical polishing method and accompanied by a proper chemical reagent to planarize the rough sketch on a wafer surface. Briefly, the Chemical Mechanical Polishing (CMP) processes involve holding and rotating a thin, flat semiconductor substrate against a wetted polishing surface under controlled chemical, pressure and temperature conditions. Once all of the parameters are properly controlled, chemical mechanical polishing can offer a smooth degree of more than 94%. The combination of mechanical and chemical removal of material during polishing results in the superior planarization of the polished surface. Therefore, semiconductor manufacturers and the suppliers of facilities and chemicals all over the world are continually investing in the development of Chemical Mechanical Polishing (CMP) technology.
During the manufacture of integrated circuits (IC) it is necessary to polish a thin wafer of semiconductor material in order to remove material and dirt from the wafer surface. Typically, a wet chemical abrasive or slurry is applied to a motor driven polishing pad while a semiconductor wafer is pressed against it in a process well known in the prior art as chemical mechanical polishing (CMP). The polishing platen is usually covered with a soft wetted material such as blown polyurethane. The polishing pad contacts the wafer surface while both wafer and pad are rotating on different axes. The rotation facilitates the transport of the abrasive containing polishing slurry between the pad and the wafer. Thus, the choice of polishing pad and slurry is determined by the material being polished, and the desired flatness of the polished surface.
Chemical Mechanical Polishing (CMP) enhances the removal of surface material over large distances and short distances by simultaneously abrading the surface while a chemical etchant selectively attacks the surface. For this purpose, Chemical Mechanical Polishing (CMP) utilizes a polishing slurry containing both an abrasive and a chemically active component. A chemical polishing slurry containing a polishing agent, such as alumina or silica, is used as the abrasive material. Necessarily, the chemical polishing slurry contains selected chemicals that etch various surfaces of the substrate during processing. Thus, the polishing effects on the wafer result from both the chemical and mechanical action.
Process reproducibility and uniformity of a Chemical Mechanical Polishing process requires periodic measurement and stringent control of the polishing slurry composition. Typically, such slurries are formulated just prior to use from an oxidant (e.g. ferric nitrate) and a particulate(e.g. alumina) dispersion. In other cases, a pre-mixed slurry may be provided. It is particularly important that careful process control is maintained over the slurry, since slurry stability may degrade over time. Part of the slurry instability can be attributed to adsorption of an oxidant onto high-surface area particles, resulting in a reduction in oxidant concentration. In addition, oxidant concentration may vary due to mixing errors and uncertainties in the original oxidant concentration used to prepare the slurry. Because oxidant concentration is one of the key parameters that control the metal removal rate (R.R) in a Chemical Mechanical Polishing process, variances in oxidant concentration may result in significant variations in the removal rates (R.R) achieved during Chemical Mechanical Polishing processes.
Consequently, it is necessary that oxidant concentration is monitored and measured during Chemical Mechanical Polishing processes. One of the conventional methods is a pre-mixing method that proceeds to mix polishing slurry (such as ssw 2000) with oxidant (such as peroxide solution; H2O2) in advance before the pre-mixing solution is carried out through titration by potassium permanganate (KMnO4), so as to measure and estimate chemical concentration after being mixed. The titration equation is shown as FIG. 1, where the liquid level concentration is obtained by estimating the potential difference at that time, the calculation of the equation is that equation.1xc3x972+equation.2xc3x975=equation.3. Both potassium permanganate (KMnO4) and the pre-mixing solution will decay in air. This method easily results in significant variations in the removal rates (R.R.) achieved during Chemical Mechanical Polishing processes. Another conventional method is the port-to-port mixing method that proceeds to mix the polishing slurry (such as ssw2000) with oxidant (such as hydrogen peroxide solution; H2O2) at the end of the polishing platen, due to the fact that the concentration of oxidant (such as peroxide solution; H2O2) is higher than one of the polishing slurry (such as ssw2000), and the quantity of oxidant (such as peroxide solution; H2O2) is less than one of polishing slurry (such as ssw2000), this method will result in pump control that is difficult. Furthermore, the conventional method cannot make up amount of oxidant at once.
In accordance with the above description, a new and improved method for controlling concentration of Chemical Mechanical Polishing (CMP) slurry is therefore necessary, so as to raise the yield and the quality of the follow-up process.
In accordance with the present invention, a method is provided for controlling the concentration of chemical mechanical polishing slurry that substantially overcomes the drawbacks of the above-mentioned problems which have arisen from the conventional methods.
Accordingly, it is an object of the present invention to provide a method for controlling concentration of chemical mechanical polishing slurry, the present invention can continuously monitor and control concentration by means of closed loop control to improve issue that can not make up amount of oxidant at once, so as to solve the above issue.
Another object of the present invention is provide that a closed loop system for controlling concentration of chemical mechanical polishing slurry, the present invention can not affect stabilization of concentration due to both of oxidant and polishing slurry can not be decayed in the ultrasonic liquid concentration analyzer. So, the present invention can determine concentration of oxidant in the volume by means of ultrasonic liquid concentration analyzer, and a fixed concentration can be obtained by it. Thus, the method of the present invention is effective in
In accordance with the present invention, a closed loop system for controlling concentration of chemical mechanical polishing slurry is disclosed. In one embodiment of the present invention, first, oxidant solution (such as hydrogen peroxide), raw slurry (such as ssw2000) and deionize water (D.I water) are transported via respective piping to flow into the blend tank, and stir mixture of the above so as to form a polishing slurry. Then, the polishing slurry is transported via piping to flow into the distribution tank. Next, the polishing slurry in the distribution tank is transported via piping to flow into the closed loop concentration control system having a ultrasonic concentration detector, a analog valve, a program logic controller (PLC) and a piping controller. The polishing slurry first flows into the ultrasonic concentration detector, so as to determine the concentration through the use of instrument. Then, the piping controller controls the polishing slurry to flow into the distribution tank via backflow piping. Furthermore, the determined data of ultrasonic concentration detector is a fluid velocity at that time, this determined data can be converted into weight percent concentration at that time by using memory data table, and converted method can be operated by manual sum or computer calculation. The converted data of weight percent concentration will be transmitted into program logic controller (PLC). Portion of the polishing slurry in the distribution tank are transmitted into the blend tank through the backflow piping. Then, the polishing slurry repeat the above steps to obtain a fixed concentration, and has been continuously proceeding the above steps to keep the fixed concentration.